The influence of the dam and sewage treatment plants on the silicon budget of the Tamagawa River

Abstract

Silicon, supplied mainly from lands, plays an important role in the primary production in the oceans. In this study we have conducted the observation of dissolved Si concentration in river water along the Tamagawa river throughout a year and spotted two zones which showed statistically significant change in the dissolved Si concentration: the Ogouchi Dam and the discharging zone of treated water from sewage treatment plants. On average, the former removed 0.8 mg/l of Si from the upper stream water containing Si at a concentration of 6.40 mg/l, whereas the latter added 1.2 mg/l of Si to the river water. The removal of Si in the dam might be attributed to diatom intake of Si in the dam, although the relation between the Si removal and its number density was insignificant. The addition of Si by the sewage treatment plants is likely due to the use of groundwater having a higher concentration of Si in the municipal areas, from which the treatment plants collected sewage, according to the consistent results of a simple mixing calculation. The annual removal of Si by the dam is estimated to be 250 ton/year and the annual load by the six sewage treatment plants is as much as 1,100 ton/year, implying that the anthropogenic input of Si along the river flow. Considering P and N concentration changes, however, N/Si and P/Si markedly increased to exceed the Redfield ratio by the addition of the treated water from the sewage treatment plants. This suggests that the anthropogenic activities along the Tamagawa river give an advert effect on diatom in the coastal area with respect to the mutual concentration of the three nutrients.